1. Field of the Invention
This invention relates to a photopolymerizable composition, particularly to the photopolymerizable composition highly sensitive to ultraviolet rays and highly stable in storage.
2. Discussion of the Background
Not a few processes for image formation by the use of a photopolymerizable composition have been known. One of those is a process for forming a relief image in which the photopolymerizable composition is prepared by combining an addition-polymerizable compound having ethylenically unsaturated double bond and a photopolymerization initiator optionally together with organic polymer binder, thermal polymerization inhibitor, coloring agent and plasticizer, the photopolymerizable composition thus obtained is applied onto the surface of a substrate with or without solvent to form a photosensitive plate provided with a layer of the photopolymerizable composition, and the photosensitive plate is then exposed through any original image to polymerize and cure the exposed areas while removing the unexposed areas by development. In another process, the photopolymerizable composition is sandwiched between two substrates, at least one of which is transparent, to form the photosensitive plate as mentioned above, and the image formation is realized after imagewise exposing on the surface of the transparent substrate followed by peeling off the same substrate, by which imagewise exposure the adhesive strength of the photopolymerizable composition is differentiated between the exposed and unexposed areas. Besides, other process of the image formation in which the toner-attracting activity of the photopolymerizable composition is changed by the exposing light is known as well.
In the foregoing processes for image formation, benzoin, benzoin alkyl ether, benzophenone, anthraquinone, benzil, Michler's ketone, etc. were conventionally employed as the photopolymerization initiator to be incorporated into those photopolymerizable composition. The sensitivity of such initiators was, however, not necessarily sufficient, and accordingly many researches had been directed to improve the sensitivity of the initiator. Thereafter, a combined initiator comprising two or more active compounds was found to considerably enhance the sensitivity. For instance, the combination of benzophenone and Michler's ketone is a typical example thereof (British Patent Specification No. 1,090,142). Various studies are still in progress to achieve further higher sensitivity, and are being reported in the literatures. In many cases, however, improved sensitivity has been found to be usually accompanied with poor stability in storage, and therefore no initiator with both higher sensitivity and higher stability in storage has yet been provided up to now.